Presented by

Tim Surtell

in association with

E LECTRONICS M ECCANO IN

Computer & Software Services Limited

Kelsey Park School Electronics Club

Lecture Outline

In this lecture we’ll look at…

• The ‘black box’ concept• Using the 555 timer IC• Three 555 timer circuits• Practical matters

After the break…

• Hands-On session:Build your 555 monostable circuit

The ‘Black Box’ of Electronics

Input(s) Process

Power

Output(s)

The Block Diagram

Input Process 1 OutputProcess 2 Process 3

555

The 555 Timer – A Black Box!

• Eight connections called ‘pins’

1

2

3

4

8

7

6

5

• Little white dot and dimple denote pin 1

The 555 Timer – A Black Box!

• The 555 uses a 3 – 18V DC power supply

0V 1

2

3

4

8 Vs

7

6

55

55

The 555 Timer – A Black Box!

• The Trigger and Reset inputs

0V 1

Trigger 2

3

Reset 4

8 Vs

7

6

55

55

Switches

0V

Switch

Trigger input

• Connecting a switch to the Trigger input: 1st attempt!

Switches

Vs

0V

Switch

Trigger input

• Connecting a switch to the Trigger input: 2nd attempt!

Bang!

Switches

• Connecting a switch to the Trigger input of the 555 using a pull-up resistor

Vs

1kΩ resistor

0V

Switch

Trigger input

The 555 Timer

• The 555 Output

0V 1

Trigger 2

Output 3

Reset 4

8 Vs

7

6

55

55

Lamps

• Connecting a lamp to the 555 Output by ‘sourcing’ current

0V

Lamp

Output

Lamps

Vs

LampOutput

• Connecting a lamp to the 555 Output by ‘sinking’ current

Light Emitting Diodes (LEDs)

0V

LED

Output

Resistor R

0.02

2-s

VR

Relays

0V

Diode

Output

RelayCoil

Vs

0V

Motor

RelaySwitch

M

The 555 Timer

• Connections for the timing components

0V 1

Trigger 2

Output 3

Reset 4

8 Vs

7 Discharge

6 Threshold

5 Offset5

55

555 Timer Circuits

Three basic circuits…

• The bistable• The monostable• The astable

Bistable Block Diagram

Trigger Input555 Output

Reset Input

• The bistable Output is stable in both states, high and low

Bistable Timing Diagram

Trigger Input

Output

Reset Input

Bistable Triggered

Bistable Reset

Monostable Block Diagram

Trigger Input 555 Output

Timing Components

• The monostable Output is stable in the low state only

• Triggering makes the Output go high for a set length of time

Monostable Timing Diagram

Trigger Input

Output

Monostable Triggered

Time period T

The Timing Components

Vs

0V

Resistor R

Capacitor C

1.1RC T

1.1C

T R

Astable Block Diagram

555 Output

Timing Components

• The astable Output is not stable at all.

It repeatedly goes high for a set time and then low for a set time

Astable Timing Diagram

Output Tmark Tspace Tmark Tspace

The Timing Components

Vs

0V

Resistor R1

Capacitor C

Resistor R2

The Timing Components

C2

0.7Rspace

T

)C2

R1

0.7(Rmark

T

)C2

2R1

(R

1.44F

Practical Matters

Three ways to build circuits…

• Breadboard for prototypes• Stripboard to produce final design• Printed Circuit Board (PCB) to produce

final design

Breadboard

Stripboard

Printed Circuit Board (PCB)

Lecture Summary

What you have learned…

• How to treat a system as a ‘black box’ with inputs, a process, and outputs

• How to break a problem into small blocks• How to use the 555 timer IC inputs and

outputs• What the bistable, monostable and

astable circuits do• How to test and construct circuits

Resistors

• Resistance is measured in Ohms (Ω)• Most resistors have four coloured bands

Band 22nd Digit

Band 3Multiplier

Band 4Tolerance

Band 11st Digit

The Resistor Colour CodeColour Band 1

1st Digit

   Band 22nd Digit

   Band 3Multiplier

   Band 4Tolerance

      Black 0 0 x 1 -

      Brown 1 1 x 10 1%

      Red 2 2 x 100 2%

      Orange 3 3 x 1,000 -

      Yellow 4 4 x 10,000 -

      Green 5 5 x 100,000 -

      Blue 6 6 x 1,000,000 -

      Violet 7 7 - -

      Grey 8 8 - -

      White 9 9 - -

      Gold - - - 5%

      Silver - - - 10%

Construction – The Resistors

The 1kΩ resistors

Brown Black Red Gold

5% 1 0 00

Construction – The Resistors

Resistors R1 and R2: 1kΩ (brown, black, red, gold)

Construction – The Resistors

The 820Ω resistor

Grey Red Brown Gold

5% 8 2 0

Construction – The Resistors

Resistor R3: 820Ω (grey, red, brown, gold)

Diodes

• Diodes only let current flow through them in one direction

1N4001 Rectifier Diode

+ -

1N4148 Signal Diode

+ -

Construction – The Diodes

Diode D1: 1N4001 (black with grey band)

Construction – The Diodes

Diodes D2 and D3: 1N4148 (orange with black band)

Light Emitting Diodes (LEDs)

• Like diodes, current only flows through them in one direction

• Three ways of identifying negative lead:

• The shortest lead

-

+

• Largest ‘triangle’ inside the LED

• Flat on the LED package

Construction – The LED

LED D4: Green 3mm LED

Construction – The 555 Timer

IC1: 555 IC Holder

IC1

Construction – The Jumper Pins

J1 and J2: ‘Smoothing’ and ‘Relay’ jumpers

Capacitors

Metallised Polyester Capacitors

Electrolytic Capacitors (polarised)

-+

Construction – Capacitors

Capacitors C1 and C2: 0.01μF (blue)

Construction – Capacitors

Capacitor C3: 220μF (smallest black)

Construction – Capacitors

Capacitor C4: 1000μF (largest black)

Construction – Variable Resistor

Variable Resistor VR1: 100kΩ

Construction – Terminal Blocks

Terminal Blocks TB1 and TB2

Construction – The Relay

Relay RL1

RL1

Time to Power Up!

+12V0V

Button

M

Motor